1. Field of the Invention
The present invention relates to an optical data recording medium and method of manufacturing the same, more particularly, relates to formation of a burst cut area (BCA) within an optical data recording medium.
2. Description of the Related Art
Optical discs, which are configured to record and reproduce data by using a laser beam, are widely-used recording media. Optical discs include read-only optical discs on which a series of emboss pits are formed in the disc manufacture process, such as CD-ROMs and DVD-ROMs, write-once optical discs, such as CD-Rs and DVD-Rs, and rewritable optical discs, such as CD-RWs, DVD-RWs and DVD-RAMs.
In general, an optical disc is designed to record and reproduce data by laser beam irradiation on a data recording layer through a transparent substrate. Within the data recording layer, a pit array or a groove is formed along a spiral recording track.
One sort of optical disc includes a burst cut area (BCA) which is a data recording region provided separately from the spiral recording track. As shown in FIG. 1A, a BCA 101 is provided at a predetermined radial position in the inner region of the optical disc. As shown in FIG. 1B, which illustrates an enlarged view of a portion A of the BCA 101, a set of radially-extending segments 101a having a low reflectance are formed within the BCA 101, and the BCA 101 records desired data as the pattern of the low-reflectance segments 101a. The BCA 101 is as sort of a bar code formed circumferentially. Hereinafter, the pattern of the low-reflectance segments 101a may be referred to as the BCA pattern and the data recorded within the BCA 101 may be referred to as the burst cut data. The BCA 101 may be provided along a part of a circumference as shown in FIG. 1A, or provided along the whole of a circumference. The burst cut data recorded in the BCA 101 typically include identifier for identifying the optical disc, optical disc type data (such as, read-only, write-once and rewritable) and copyright control data.
Referring back to FIG. 1B, the low-reflectance segments 101a are circumferentially arranged at a predetermined radial position of the optical data recording medium. The segments 101a have a fixed length along the radial direction, and the segments 101a are patterned along the circumference, depending on data to be recorded. It should be noted that a recording track, which is to be tracked by the laser spot, is not provided within the BCA 101. The SCA 101 is scanned by the laser spot along the circumference direction indicated by the arrow S, which is same as the disc rotation direction. The reflected light intensity changes depending on the BCA pattern (that is, the pattern of the low-reflectance segments 101a and the high-reflectance portions 101b), and the change in the reflected light intensity is detected as a signal. This signal is used for reproducing the data recorded in the BCA 101.
Compared with data recording with pits, one advantage of the data recording onto the BCA is that the use of the BCA allows recording different data onto different optical data recording media. For example, the use of BCA allows recording copyright control data onto each optical data recording medium. One approach of the BCA formation within a read-only optical data recording medium is to partially remove a metal reflective film within a data recording layer by irradiation of a strong laser beam, and to thereby form low-reflection segments within the read-only optical data recording medium. This approach is inferior in the fineness, compared with the formation of pits. For example, the low-reflectance segments 101a actually have a width of about 10 μm or more, and the largest interval between adjacent low-reflectance segments 101a is 100 μm or more.
Recently, optical discs with two data recording layer have become commercially available, which are adapted to record and reproduce data through one light-incident surface. FIG. 2 illustrates an example of the section structure of an optical data recording medium with two data recording layers. The optical data recording medium of FIG. 2, denoted by the numeral 1, incorporates a zero-th data recording layer 10 and a first data recording layer 11. Each of the data recording layers 10 and 11 includes a film stack including a phase-change recording film. The data recorded onto the data recording layers 10 and 11 are reproduced by using a focused beam irradiated through the light-incident surface 1A.
The distance between the zero-th data recording layer 10 and the first data recording layer 11 is preferably in the range in which the aberration of the focused beam passing through a transparent substrate 14 of the optical data recording medium 1 is not largely changed. Therefore, the distance is adjusted to about 50 μm in a DVD-ROM, for example. In this case, the diameter of the focused beam at the zero-th data recording layer 10 is about 40 μm when the focused beam is focused on the first data recording layer 11. This value is in the same order as intervals of the low-reflectance segments 101a within the BCA 101.
Therefore, when the focused beam is focused on one of the data recording layers 10 and 11, the focused beam experiences considerable interference caused by the other of the data recording layers 10 and 11. In order to avoid this, the BCA is formed within only one of the two data recording layers 10 and 11.
In this case, it would be advantageous if the BCA is formed within predetermined one of the data recording layers 10 and 11; this effectively reduces the time duration necessary for a recording apparatus or a reproducing apparatus to identify the optical data recording medium. When the data recording layer with the BCA is unknown, it is necessary to access both of the data recording layers 10 and 11 to find the BCA. When the BCA is formed within fixed one of the data recording layers 10 and 11, on the other hand, the data recorded within the BCA is reproduced in a short time through accessing only one of the data recording layers in which the BCA is formed.
When the optical data recording medium 1 is a read-only medium with two data recording layers, the first data recording layer 11 incorporates a metal reflection layer to provide a sufficient amplitude for a reproduced signal. As shown in FIG. 3A. a BCA is formed for such read-only medium through irradiating a laser beam through another light-incident surface is, which is opposed to the light-incident surface 1A used for the data recording and reproduction; such technique is disclosed in Japanese Laid Open Patent Application No. JP-A 2005-135569 and the corresponding U.S. Patent Application Publication NO. 2005/0078594. The metal reflective film within the first data recording layer 11 is partially removed by the laser beam to thereby form the BCA. Data reproduction from the first data recording layer 11 is achieved with a laser beam passing through the zero-th data recording layer 10. Therefore, the first data recording layer 11 is formed to have an increased reflectance sufficient for providing a necessary amplitude for the reproduced signal. Typically, the metal reflective film within the first data recording layer 11 has an increased film thickness so that the transmittance of the metal reflective film is reduced down to 10% or less.
When the transmittance of the zero-th data recording layer 10 is T0 and the individual reflectance of the first data recording layer 11 is R1, the total reflectance R of the structure composed of the zero-th and first data recording layers 10 and 11 is represented by the following formula:R=R1×T0.The individual reflectance R1 of the first data recording layer 11 should be increased up to 60% or more for increasing the total reflectance R up to 15% or more, when the transmittance T0 of the zero-th data recording layer 10 is 50%. When a silver reflective film is incorporated within the first data recording layer 11, for example, the silver reflective film is required to have a thickness of about 50 nm for achieving a reflectance of 60%. In this case, the first data recording layer 11 has a transmittance of about 10%. Therefore, when a laser beam is radiated from the side of the first data recording layer 11, the laser beam seldom transmits through the first data recording layer 11. The reflection beam from the zero-th data recording layer 10 is not detected, and this allows the focus servo control to surely focus the laser beam on the first data recording layer 11.
As for a rewritable optical disc, the BCA is formed through irradiating a laser beam on a phase-change film to change the optical characteristics thereof, instead of removing the metal reflective film; this process is similar to that of recording user data onto the rewritable optical disc. For a read-only medium in which each data recording layer consists of a single metal film, the BCA is formed through partially removing the metal reflective film by using a laser beam. However, such process is not applicable to rewritable optical disc, in which each data recording layer consists of a film stack. It is difficult to finely remove all the desired portions of the film stack by laser irradiation. The BCA formation within a rewritable optical disc is achieved by using an approach similar to user data recording.
For the BCA formation of a read-only medium, a laser beam is irradiated from the side of the first data recording layer 11. This allows surely focusing the laser beam on the first data recording layer 11. The formation of a BCA within a rewritable multilayer optical disc, on the other hand, requires irradiating a laser beam from the side of zero-th data recording layer 10. The reason is as follows: Since the first data recording layer 11 incorporates a metal reflective film having a low transmittance, the laser beam irradiated from the side of the first data recording layer 11 is hard to transmit the first data recording layer 11. Therefore, the intensity of the laser beam transmitted through the first data recording layer 11 is not so high that the temperature of the phase-change film is increased to change the optical characteristics thereof. This implies that the BCA formation requires irradiating a laser beam from the zero-th data recording layer 10, which has a high transmittance, to thereby increase the temperature of the phase-change recording film within the first data recording layer 11.
However, the laser beam is not necessarily focused on the first data recording layer 11, when the laser beam is irradiated from the side of the zero-th data recording layer 10. For a read-only medium, in which the first data recording layer 11 has an extremely low transmittance, a change of a focus error signal 7, which is often called the S-shaped curve, is observed only once as shown in FIG. 3A, when the laser beam is irradiated from the side of the first data recording layer 11. As for a rewritable optical disc, on the other hand, two S-shaped curves are observed as shown in FIG. 3B, one of which is caused by the zero-th data recording layer 10, and the other is caused by the first data recording layer 11. The laser beam is controlled to be focused on the first data recording layer 11 on the basis of one obtained from the first data recording layer 11 out of the two S-shape curves. However, the laser beam may be erroneously focused on the zero-th data recording layer 10, due to the S-shape curve obtained from the zero-th data recording layer 10. When the laser beam is erroneously focused on the zero-th data recording layer 10, the BCA is undesirably formed within the zero-th data recording layer 10. A multilayer optical disc in which a BCA is formed within the zero-th data recording layer 10 is an unusable defective product.
Correspondingly, the laser beam is required to be focused on the zero-th data recording layer 10 when a BCA is formed within the zero-th data recording layer 10; however, the laser beam may be erroneously focused on the first data recording layer 11, and a BCA may be accidentally formed within the first data recording layer 11.
As described above, fixing the data recording layer within which the BCA is formed effectively reduce the time duration necessary for medium identification; however, the conventional approach suffers from a difficulty in surely forming a BCA within a desired data recording layer in a rewritable data recording medium with multiple data recording layers. A rewritable data recording medium with multiple data recording layers in which a BCA is formed within an undesired data recording layer is an unusable defective product.
Japanese Laid Open Patent Application No. WO2002/037483 and the corresponding U.S. Patent Application Publication No. 2003/0076775 disclose a technique for forming a BCA within an optical disc with two data recording layers. In this technique, first and second data recording layers are formed on first and second substrates, respectively, and then a BCA is formed only within the first data recording layer. This is followed by bonding the first and second substrates by an intermediate layer. Japanese Laid Open Patent Application No. JP-A 2002-313031 and the corresponding U.S. Patent Application Publication No. 2002/0150031 disclose a similar technique.